Nonionic and anionic surfactants are important constituents in many applications. Both aromatic and aliphatic sulfates and sulfonates are an important group of anionic surface-active agents used extensively in a number of industrial applications. These include operations in drilling for and recovery of crude oil; emulsifiers for pesticides used in crop protection; in shampoos and creams for personal care; bar soaps; laundry detergents; dishwashing liquids, hard surface cleaners; emulsifiers for emulsion polymerization systems; lubricants; wetting agents; and dispersants in a variety of specialized industrial applications.
The surfactants used in cleaning applications are designed to remove a wide variety of soils on fabrics and hard surfaces. Surfactants in this application have a balance of particulate soil removal and grease and oily soil removal characteristics. Especially in detergent compositions for cleaning fabrics, the surfactants used should have the ability to remove a broad spectrum of soil types.
In many cases, however, a surfactant which exhibits high detergency power will be poorly soluble in cold water. For example, surfactants present in laundry powder detergents should dissolve completely in a relatively short time interval under whatever wash temperature and agitation conditions are employed in the wash cycle chosen by the consumer. Undissolved detergent not only fails to provide cleaning benefits, but also may become entrapped in the laundry articles and remain behind as a residue either in the machine or on the garments themselves. The problem of dispersion and solubilization in the wash cycle are made worse under conditions of cold water washing especially at or below about 50° F. (10° C.). Lower wash temperatures are becoming ever increasing factors in today's wash loads as both energy conservation and increased use of highly colored, delicate fabrics lead to wash conditions that make powders difficult to dissolve.
In contrast to nonionic surfactants, which exhibit inverse solubility behavior and which, by virtue of hydrogen bridge bonds, show better solubility in cold water than in warm water, anionic surfactants show conventional behavior, i.e. their solubility increases more or less linearly with the temperature until the solubilized product is reached. The surfactant employed, whether anionic or nonionic, should be designed to remain homogeneous in the wash media at cold water washing temperatures to optimize the cleaning performance of the surfactant. Accordingly, surfactants with the ability to remove sebum types of soil and which have low Krafft point temperatures are desirable.
Surfactants which have good washing and cleaning performance have low Krafft temperatures. The Krafft temperature refers to the temperature at which the solubility of an anionic surfactant undergoes a sharp, discontinuous increase with increasing temperature. The solubility of an anionic surfactant will increase slowly with an increase in temperature up to the temperature point at which the solubility exhibits an extremely sharp rise. The temperature corresponding to the sharp rise in solubility is the Krafft temperature of anionic surfactant. At a temperature approximately 4° C. above the Krafft temperature, a solution of almost any composition becomes a homogeneous phase. Further, the Krafft temperature is a useful indicator of detergency performance because at and above the Krafft temperature, surfactants begin to form micelles instead of precipitates, and below the Krafft temperature point, surfactants are insoluble and form precipitates. At the Krafft point temperature, the solubility of a surfactant becomes equal to its critical micelle concentration, or CMC. The appearance and development of micelles are important since certain surfactant properties such as foam production depend on the formation of these aggregates in solution.
Each type of surfactant will have its own characteristic Krafft temperature point. In general, the Krafft temperature of a surfactant will vary with the structure and chain length of the hydrophobic hydrocarbyl group and hydrophilic portion of the molecule. Krafft temperature for ionic surfactants is, in general, known in the art. See, for example, Myers, Drew, Surfactant Science and Technology, pp. 82-85, VCH Publishers, Inc. (New York, N.Y., USA), 1988 (ISBN 0-89573-399-0), and K. Shinoda in the text “Principles of Solution and Solubility”, translation in collaboration with Paul Becher, published by Marcel Dekker, Inc. 1978 at pages 160-161, each of which is incorporated by reference herein in its entirety.
A surfactant which exhibits a high Krafft point is generally insufficient in detergency and foaming power. Since the Krafft point is a factor having an influence on the surface activating capacities of a surfactant, at temperatures lower than the Krafft point, surface-activating capacities such as detergency, foaming power and emulsifying power begin to deteriorate, and the surfactant may precipitates on the fabric. Thus, the surfactant should desirably possess a low Krafft point, especially in light of current performance requirements in cold water washing temperatures.
However, even surfactants with good detergency and high cold water solubility limits, as shown by their low Krafft point temperatures, may nevertheless leave behind precipitates on the surface to be cleaned if the surfactant is not tolerant to the concentration of electrolytes (typically magnesium and calcium) present in the aqueous washing medium. The electrolyte of most concern in wash water is calcium due to its high concentration in many aqueous media and its ability to exchange with the soluble sodium cation on sulfated surfactants to form an insoluble calcium salt of the sulfated surfactant, which precipitates out onto the substrate to be cleaned as a particle or film. The hardness of the water, or concentration of calcium and other electrolytes in water, will vary widely depending on the purification method and efficiency of water treatment plants which dispense water to the consumer of the detergent or cleaning composition. Accordingly, there remains a need to provide a surfactant which is tolerant to high concentrations of calcium so as to provide a cleanser which performs as expected in a wide variety of aqueous media.
Due to constraints on water consumption, especially in locations where the supply of drinking water to a population is limited, inadequate, or expensive, there is a desire to employ unprocessed or lightly processed water having a high concentration of saline as a wash media. In particular, there exists a need in some locations to use sea water or brackish water which is unprocessed or lightly processed as the aqueous media for many applications outside of drinking water, such as dishwashing and laundry water. The need to provide for a surfactant which is tolerant to high concentrations of electrolytes, such as calcium, is readily apparent if one must wash or clean a substrate in sea water or brackish water. Thus, there also exists a desire to find a surfactant composition, which is so highly tolerant to calcium that it is suitable for use in seawater or brackish as a cleansing agent.
It would also be desirable to manufacture a surfactant which can be easily and economically stored and transported. Polyoxyethylene nonionic linear alcohol surfactants, especially those containing from 3 or more ethylene oxide units, are solid or waxy products at ambient conditions (25° C. and 1 atm). Since these waxy or solid products cannot be pumped at ambient conditions, they must first be melted into the liquid phase and kept as a liquid during offloading and feeding into a reaction vessel or a blend tank. Further, the waxy and solid polyoxyethylene linear alcohols must be shipped and/or transported in drums, which take up more warehouse space than liquid storage tanks. It would be desirable to produce a polyoxyalkylene surfactant which is flowable and pumpable at ambient conditions, and yet more desirable to produce such an surfactant which is flowable and pumpable in cold climates where temperatures drop to 0° C.